Communication systems use oscillators to process various signals used in the communication process. In a typical communication system, an information signal (e.g., a television program) is modulated onto a high frequency carrier signal to facilitate transmission of the signal. Such modulation may be accomplished, for example, by varying the phase of the carrier signal according to the information in the signal. Through the use of different carrier signals of different frequencies, many information signals may be simultaneously broadcast in a communication system. A receiver in the communication system typically uses an oscillator to separate the information signal from the carrier signal. Moreover, to accommodate the various carrier frequencies, the oscillator may be controllable so that the frequency of the signal generated by the oscillator may be varied as necessary.
In practice, oscillators do not generate a perfect signal; that is, a signal with precisely fixed frequency, amplitude and phase characteristics. Rather, due to a variety of factors including internal noise generated by the electronic components of the oscillator, the frequency, amplitude and phase of an oscillator will vary over a fairly narrow range over time.
Low-noise electronic oscillators are crucial to the effective operation of communication receivers and transmitters that use superheterodyne techniques and modulation formats that involve the phase of signal. As indicated above, amplitude and phase modulation are present in all real oscillators; however the nature of oscillators is such that for frequencies approaching the center frequency of the oscillation, the noise is dominated by phase fluctuations. Fluctuations in the phase of the transmitter or receiver oscillators corrupt the intended phase modulation of the signal in systems which use phase or frequency modulation. Phase noise may also reduce the ability of a radio system to reject or discern unwanted channels close in frequency to the desired channel.
Various techniques exist in the art to reduce phase noise such as phase locking to a less-noisy reference signal and the use of tracking phase-locked loops in the receiver. Nevertheless, system performance is often limited by the quality of available oscillators at practical levels of cost, size, and power dissipation.
Oscillators also may suffer from a significant level of noise due to up-conversion of flicker (1/f) noise from low frequencies to near the center frequency of the oscillator. Flicker noise occurs at low frequencies in nearly all electronic components.
The invention is directed to techniques for reducing noise in electronic oscillators. A folding technique is described for controlling circuit parameters in an electronic oscillator by separately controlling a plurality of circuit elements that affect those parameters. Through the use of a plurality of circuit elements, substantial noise reduction can be achieved. In addition, the use of a folding technique facilitates making smooth transitions when sequentially activating circuit elements. Thus, the folding technique described herein ensures that the combined effect of the adjustable circuit elements results in a relatively smooth adjustment of the circuit parameters.
In one embodiment the oscillator includes a plurality of varactors in the oscillator""s tank circuit. The frequency of the oscillator is controlled, in part, by varying the voltage across the varactors. By using several varactors having relatively small capacitance instead of one varactor having a relatively large capacitance, flicker noise can be substantially reduced in the oscillator. In addition, a folding circuit is utilized to control the varactors so that each varactor is activated in a substantially sequential manner, relative to one another, such that the overall change in capacitance in the tank circuit occurs in a relatively smooth manner, without substantial transient effects.
In one embodiment, the folding technique takes advantage of a sigmoidal response characteristic of the varactor components. This sigmoidal response characteristic has the effect that the capacitance of the varactor changes in a substantially linear manner when a particular range of voltages are applied across the varactor. Outside that voltage range, however, the capacitance of the varactor remains relatively constant. By properly folding the variable capacitance of each of these varactors into the tank circuit of the oscillator, the invention achieves very low noise characteristics in a controllable oscillator.
In another embodiment of the invention, a folding circuit sequentially activates varactors in the circuit such that when the circuit is ready to fold a new varactor into the circuit, the folding circuit couples a fixed capacitance to the tank circuit to replace a varactor that is then isolated from the circuit. Again, the folding circuit facilitates smooth transitions in the total capacitance of the oscillator tank circuit.
Accordingly, an oscillator constructed according to the invention includes at least one folding circuit responsive to at least one control signal, the folding circuit generating a plurality of varactor control signals; and at least one oscillator circuit comprising a plurality of varactors responsive to the plurality of control signals.